Zone refinement of liquid-phase sintered tungsten alloys



United States Patent 3,372,024 ZONE REFHNEMENT F LIQUID-PHASE SINTEREDTUNGSTEN ALLGYS Niranjan M. Parilch, Chicago, Ill., assignor, by mesneassignments, to the United States of America as represented by theSecretary of the Army No Drawing. Filed Nov. 10, 1966, Ser. No. 594,3174 Claims. (Cl. 75-84) ABSTRACT OF THE DISCLOSURE This process is atreatment of a sintered tungsten alloy for a matrix which utilizes zonerefining for the removal of the low melting matrix while depositingtherefrom the contained tungsten on the sintered tungsten for theproduction of practically pure tungsten.

The invention described herein may be manufactured and used by or forthe Government, for governmental purposes, without the payment to me ofany royalty there on.

This invention relates to the refinement of tungsten alloys in animproved manner thereby circumventing the difiicult, wasteful andexpensive operations presently employed in current industrialoperations.

The treatment of this process is based on the techniques of powdermetallurgy and it is therefore the object of this invention to producepure sheets or bars of tungsten in this manner.

It is a further object of this invention to utilize zone refining forthe removal of the liquid phase of a sintered tungsten alloy for theproduction of practically pure tungsten in which the microstructureindicates that the tungsten grain boundaries are clean and the grainsare characteristically equiaxed.

This process employs liquid phase sintering of high tungsten alloysusing a nickel-iron alloy for a matrix and subsequently removing the lowmelting nickel-iron alloy This treatment consists in forming compactsfrom powders of compositions ranging from 9099% by weight of tungstenand the balance nickel and iron in a ratio of 5.5 to 8.2, respectively.These compacts in the form of long bars are sintered at temperaturesabove 1475 C. in a protective atmosphere such as hydrogen, vacuum orargon. Densifiication takes place first by liquification of the lowmelting nickel-iron alloy with an increasing temperature andsubsequently decreasing the temperature for the solution andreprecipitation of tungsten in this liquid.

The tungsten-nickel-iron bars prepared in this way are then movedthrough a narrow hot zone at a controlled rate of speed, for example oneinch per hour in a protective atmosphere. Maintaining this hot zoneabove the nickel-iron matrix causes the liquid to migrate out of thesintered bar and reprecipitates all the excess tungsten on the tungstengrains.

Tests were made in which a 98% tungsten-2% (6Niz4Fe) alloy was sinteredat 1560 C. in an atmosphere of hydrogen and microstructures of thissintered alloy showed tungsten grains embedded in a semi-continuousmatrix of nickel-iron rich alloys; this sintered alloy was passedthrough the hot zone at a temperature of 1550 C. at a nominal rate ofmotion of one inch per hour, for the zone refinement of the liquidphase. Microstructures of the refined alloy showed that the liquid phaseof the matrix had been removed and that the tungsten boundaries wereclean and the grains characteristically equiaxed.

A further test was made employing an alloy having an 3,372,024 PatentedMan 5, 1 968 initial sintered composition of 95% tungsten and 5%(6Niz4Fe) and when zone refined at 153 0 C. in a vacuum themicrostructures showed the same results.

Thus, this treatment employing zone refining of liquid phase of sinteredtungsten alloys as a means of producing pure tungsten without thedifiicult high temperature operations normally employed and with theachievement of claim boundaries between tungsten grains and the equiaxedgrain structure is indicative of a very desirable degree of grainrefinement.

What is claimed is: 1. A process for the production of pure tungstenfrom a sintered iron-nickel-tungsten alloy comprising,

maintaining the sintered material above the melting point of theiron-nickel matrix by zone refining so as to release the tungsten heldin the matrix not in pure form and thereby depositing the tungsten freedfrom the matrix on the pure tungsten grains by draining the moltenmatrix metal, whereby practically pure tungsten is produced having amicrostructure indicating clean boundaries and characteristicallyequiaxed. 2. A process for the production of bars of pure tungstencomprising,

forming compacts of long bars prepared from powdered -99% tungstenalloys using a powdered ironnickel alloy as a matrix, sintering saidbars at a temperature above 1475 C. in

a protective atmosphere, maintaining the sintered material above themelting point of the iron-nickel matrix by zone refining so as torelease the tungsten held in the matrix not in pure form and therebydepositing the tungsten freed from the matrix on the pure tungstengrains by draining the molten matrix metal, whereby the zone refining ofthe liquid phase of a sintered alloy removes the impurities of thematrix to produce practically pure tungsten. 3. A process for theproduction of bars of pure tungsten comprising,

forming compacts of long bars prepared from powdered 90-99% tungstenalloys using a powdered iron nickel alloy as a matrix, sintering saidbars at a temperature above 1475 C.

in a protective atmosphere, densifying the sintered material byincreasing the temperature above the melting point of the iron-nickelalloy to free the tungsten in the matrix, and subsequently decreasingthe temperature for the solution and precipitation of tugnsten in thisliquid and zone refining the densified material by passing the densifiedmaterial slowly through a hot zone at 15 50 C. in a protectiveatmosphere to permit the removal of the molten iron-nickel matrix,whereby the zone refining of the liquid phase of a sintered alloyremoves the impurities of the matrix to produce practically puretungsten. 4. A process for the production of bars of pure tungstencomprising,

forming compacts of long bars from a powdered tungsten alloy containing-99% tungsten, 15% of a nickel-iron alloy having a ratio of 6Niz4Feusing the nickel iron alloy as a matrix, sintering said bars at atemperature above 147 5 C. in

a hydrogen atmosphere, zone refining the sintered material by passingthe sintered material through a hot zone at approximately 1530 C. in avacuum at a rate of approximately one inch an hour for refining byfreeing the tungsten 4 combined in the matrix for deposit on the freetung- References Cited r 2il 1 6 3i1t6n matrix metal while in the hotUNITED STATES PATENTS in Zone, g 3,218,154 11/1965 Sell et a1 75 10whereby practically pure tungsten is obtained as a product of the zonerefining of the liquid phase of 5 DEWAYNE RUTLEDGE "nary Exammer thesintered alloy. M. I SCOLNICK, Assistant Examiner.

